Method for making a semiconductor device having a high-k gate dielectric

ABSTRACT

A method for making a semiconductor device is described. That method comprises forming a high-k gate dielectric layer on a substrate. After removing impurities from that layer, and increasing its oxygen content, a gate electrode is formed on the high-k gate dielectric layer.

FIELD OF THE INVENTION

The present invention relates to methods for making semiconductordevices, in particular, semiconductor devices that include high-k gatedielectric layers.

BACKGROUND OF THE INVENTION

MOS field-effect transistors with very thin silicon dioxide based gatedielectrics may experience unacceptable gate leakage currents. Formingthe gate dielectric from certain high-k dielectric materials, instead ofsilicon dioxide, can reduce gate leakage. Such a dielectric may not,however, be compatible with polysilicon—the preferred material formaking the device's gate electrode.

If such a high-k film comprises an oxide, it may manifest oxygenvacancies and excess impurity levels. Oxygen vacancies may permitundesirable interaction between the high-k film and the gate electrode.When the gate electrode comprises polysilicon, such interaction mayafter the electrode's workfunction or cause the device to short throughthe dielectric. If the process for forming the high-k film uses a metalchloride precursor, residual chlorine may adversely affect the device'selectrical properties.

Accordingly, there is a need for an improved process for making asemiconductor device that includes a high-k gate dielectric. There is aneed for such a process for forming a very thin high-k gate dielectricthat improves the interface between the high-k film and the gateelectrode by minimizing oxygen vacancies in the high-k film. There is aneed for a process for forming a high-k gate dielectric with acceptableimpurity levels. The method of the present invention provides such aprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1 c represent cross-sections of structures that may be formedwhen carrying out an embodiment of the method of the present invention.Features shown in these figures are not intended to be drawn to scale.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A method for making a semiconductor device is described. That methodcomprises forming on a substrate a high-k gate dielectric layer, thenremoving impurities from the high-k gate dielectric layer, andincreasing the oxygen content of the high-k gate dielectric layer. Agate electrode is then formed on the high-k gate dielectric layer. Inthe following description, a number of details are set forth to providea thorough understanding of the present invention. It will be apparentto those skilled in the art, however, that the invention may bepracticed in many ways other than those expressly described here. Theinvention is thus not limited by the specific details disclosed below.

In an embodiment of the method of the present invention, high-k gatedielectric layer 110 is formed on substrate 100, as shown in FIG. 1a.Substrate 100 may comprise a bulk silicon or silicon-on-insulatorsubstructure. Alternatively, substrate 100 may comprise othermaterials—which may or may not be combined with silicon—such as:germanium, indium antimonide, lead telluride, indium arsenide, indiumphosphide, gallium arsenide, or gallium antimonide. Although severalexamples of materials from which substrate 100 may be formed aredescribed here, any material that may serve as a foundation upon which asemiconductor device may be built falls within the spirit and scope ofthe present invention.

When substrate 100 comprises a silicon wafer, the wafer may be cleanedbefore forming high-k gate dielectric layer 110 on its surface. To cleanthe wafer, it may initially be exposed to a dilute hydrofluoric acid(“HF”) solution, e.g., a 50:1 water to HF solution. The wafer may thenbe placed in a megasonic tank, and exposed first to a water/H₂O₂/NH₄OHsolution, then to a water/H₂O₂/HCl solution. The water/H₂O₂/NH₄OHsolution may remove particles and organic contaminants, and thewater/H₂O₂/HCl solution may remove metallic contaminants.

After that cleaning treatment, high-k gate dielectric layer 110 may beformed on substrate 100, generating the FIG. 1a structure. High-k gatedielectric layer 110 comprises a material with a dielectric constantthat is greater than the dielectric constant of silicon dioxide.Dielectric layer 110 preferably has a dielectric constant that is atleast about twice that of silicon dioxide, i.e., a dielectric constantthat is greater than about 8. Materials that may be used to make high-kgate dielectrics include: hafnium oxide, hafnium silicon oxide,lanthanum oxide, zirconium oxide, zirconium silicon oxide, titaniumoxide, tantalum oxide, barium strontium titanium oxide, barium titaniumoxide, strontium titanium oxide, yttrium oxide, aluminum oxide, leadscandium tantalum oxide, and lead zinc niobate. Particularly preferredare hafnium oxide, zirconium oxide, titanium oxide, and aluminum oxide.Although a few examples of materials that may be used to form dielectriclayer 110 are described here, that layer may be made from othermaterials that serve to reduce gate leakage.

High-k gate dielectric layer 110 may be formed on substrate 100 using aconventional deposition method, e.g., a conventional chemical vapordeposition (“CVD”), low pressure CVD, or physical vapor deposition(“PVD”) process. Preferably, a conventional atomic layer CVD process isused. In such a process, a metal oxide precursor (e.g., a metalchloride) and steam may be fed at selected flow rates into a CVDreactor, which is then operated at a selected temperature and pressureto generate an atomically smooth interface between substrate 100 anddielectric layer 110. The CVD reactor should be operated long enough toform a layer with the desired thickness. In most applications,dielectric layer 110 should be less than about 60 angstroms thick, andmore preferably between about 5 angstroms and about 40 angstroms thick.

As deposited, high-k gate dielectric layer 110 may be incompatible withpolysilicon due to the presence of unacceptable numbers of oxygenvacancies and excess impurity levels. For example, when a metal chlorideprecursor is used to form high-k gate dielectric layer 110, chlorine maypermeate through that layer. A transistor with a high-k gate dielectriclayer that includes a significant amount of chlorine may exhibitinferior electrical properties. In the method of the present invention,impurities are removed from high-k gate dielectric layer 110 and thatlayer's oxygen content is increased, after it is formed on substrate100. After removing impurities and increasing the oxygen content, theresulting high-k gate dielectric layer 110 may be compatible withpolysilicon, or other materials that may be used to form the gateelectrode.

In an embodiment of the present invention, a wet chemical treatment isapplied to high-k gate dielectric layer 110 to remove impurities fromthat layer and to increase that layer's oxygen content. The wet chemicaltreatment may comprise exposing high-k gate dielectric layer 110 to asolution that comprises a source of hydroxide at a sufficienttemperature for a sufficient time to remove impurities from high-k gatedielectric layer 110 and to increase the oxygen content of high-k gatedielectric layer 110. That solution preferably has a pH of at leastabout 7, and more preferably a pH of between about 11 and about 13. Thesource of hydroxide may comprise, for example, deionized water, hydrogenperoxide, ammonium hydroxide, and/or a tetraalkyl ammonium hydroxide,such as tetramethyl ammonium hydroxide (“TMAH”). The appropriate timeand temperature at which high-k gate dielectric layer 110 is exposed maydepend upon the source of hydroxide that is included in the solution,and upon the desired thickness and other properties for high-k gatedielectric layer 110.

When high-k gate dielectric layer 110 is exposed to a solution thatconsists essentially of deionized water, high-k gate dielectric layer110 should be exposed to such a solution for at least about one minuteat a temperature of at least about 35° C. In a particularly preferredembodiment, high-k gate dielectric layer 110 may be exposed to such asolution for about 20 minutes at a temperature of about 40° C.

When high-k gate dielectric layer 110 is exposed to a hydrogen peroxidebased solution, an aqueous solution that contains between about 2% andabout 30% hydrogen peroxide by volume may be used. That exposure stepshould take place at between about 15° C. and about 40° C. for at leastabout one minute. In a particularly preferred embodiment, high-k gatedielectric layer 110 is exposed to an aqueous solution that containsabout 6.7% H₂O₂ by volume for about 10 minutes at a temperature of about25° C.

When high-k gate dielectric layer 110 is exposed to an ammoniumhydroxide based solution, an aqueous solution that contains betweenabout 2% and about 30% ammonium hydroxide by volume may be used. Thatexposure step should take place at between about 15° C. and about 90° C.for at least about one minute. In a particularly preferred embodiment,high-k gate dielectric layer 110 is exposed to an aqueous solution thatcontains about 15% NH₄OH by volume for about 30 minutes at a temperatureof about 25° C.

When high-k gate dielectric layer 110 is exposed to a hydrogenperoxide/ammonium hydroxide based solution, an aqueous solution thatcontains between about 1% and about 10% hydrogen peroxide by volume, andbetween about 1% and about 10% ammonium hydroxide by volume, may beused. That exposure step should take place at between about 15° C. andabout 40° C. for at least about one minute. In a particularly preferredembodiment, high-k gate dielectric layer 110 is exposed to an aqueoussolution that contains about 4.2% H₂O₂ by volume and about 4.2% NH₄OH byvolume for about 10 minutes at a temperature of about 25° C.

When high-k gate dielectric layer 110 is exposed to a TMAH basedsolution, an aqueous solution that contains between about 2% and about30% TMAH by volume may be used. That exposure step should take place atbetween about 15° C. and about 90° C. for at least about one minute. Ina particularly preferred embodiment, high-k gate dielectric layer 110 isexposed to an aqueous solution that contains about 25% TMAH by volumefor about 2 minutes at a temperature of about 80° C.

While high-k gate dielectric layer 110 is exposed to a solution thatcomprises a source of hydroxide, it may be desirable to apply sonicenergy at a frequency of between about 10 KHz and about 2,000 KHz, whiledissipating at between about 1 and about 10 watts/cm². In a preferredembodiment, sonic energy may be applied at a frequency of about 1,000KHz, while dissipating at about 5 watt/cm².

When the wet chemical treatment of the present invention is applied tohigh-k gate dielectric layer 110, the chlorine content of that layer maydecrease by at least about 80 percent, and perhaps as much as 90percent. In addition to providing a source of hydroxide, a hydrogenperoxide containing solution may act as an oxidizer. When such asolution is used, the oxygen content of layer 110 may increase by atleast about 10 percent. (Other sources of hydroxide, which serve toreplace impurities with hydroxyl groups, may also increase layer 110'soxygen content to some extent.)

When high-k gate dielectric layer 110 is exposed to the solutionsdescribed above, that layer may be partially etched. Using the method ofthe present invention to reduce the thickness of layer 110, whilesimultaneously increasing that layer's oxygen content and reducing itschlorine content, may be particularly advantageous when a slightlythinner layer is desired. At least about 10% of high-k gate dielectriclayer 110 (and perhaps as much as 30% or more) may be partially etched,when that layer is exposed to certain of these solutions. When exposinghigh-k gate dielectric layer 110 to the above described solutions, anegligible amount of oxide, if any, may grow on substrate 100. In apreferred embodiment, less than about 3 angstroms of oxide, if any, willgrow on substrate 100, when layer 110 is exposed to these solutions.

In the method of the present invention, a single wet chemical treatmentmay be applied after high-k gate dielectric layer 110 has beencompleted. Alternatively, an iterative approach may be applied, in whicha series of deposition steps alternate with wet chemical treatmentsteps. In such an iterative process, a second high-k gate dielectriclayer is formed after the initial wet chemical treatment, and thatsecond high-k gate dielectric layer is exposed to a second solution thatincludes a source of hydroxide. A third layer may then be formedfollowed by a third wet chemical treatment, and so on, until the desiredthickness for the high-k gate dielectric layer is achieved.

Although a few examples of wet chemical treatments that may be used toremove impurities from high-k gate dielectric layer 110, and to increasethat layer's oxygen content, are described here, other treatments thatserve to modify high-k gate dielectric layer 110 in that way may be usedinstead, as will be apparent to those skilled in the art. Examplesinclude exposing high-k gate dielectric layer 110 to aqueous solutionsthat contain ozone, or to other solutions that contain other types ofoxidizing and/or hydrolyzing agents.

After removing impurities from high-k gate dielectric layer 110, andincreasing that layer's oxygen content, a gate electrode is formed onlayer 110. In a preferred embodiment, the gate electrode may be formedby initially depositing polysilicon layer 120 on high-k gate dielectriclayer 110—generating the FIG. 1b structure. Polysilicon layer 120 may bedeposited using conventional methods and preferably is between about 500angstroms and about 4,000 angstroms thick. After etching both layers 120and 110 to form the FIG. 1c structure, using conventional techniques,additional steps that are generally used to complete the gate electrode(e.g., forming a silicide (not shown) on the upper part of etchedpolysilicon structure 130) may be applied. As such steps are well knownto those skilled in the art, they will not be described in more detailhere. Although the gate electrode preferably comprises polysilicon, itmay alternatively be formed from various metals with which the abovedescribed high-k gate dielectrics may be used.

The method of the present invention may enable a high-k gate dielectricto be used with a polysilicon-based gate electrode. By removingimpurities from high-k gate dielectric layer 110 and increasing thatlayer's oxygen content, the dielectric layer's surface and electricalproperties may be enhanced, which may render it suitable for use withpolysilicon and other gate electrode materials.

Although the foregoing description has specified certain steps andmaterials that may be used in the method of the present invention, thoseskilled in the art will appreciate that many modifications andsubstitutions may be made. Accordingly, it is intended that all suchmodifications, alterations, substitutions and additions be considered tofall within the spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A method for making a semiconductor devicecomprising: forming a high-k gate dielectric layer on a substrate, thehigh-k gate dielectric layer comprising impurities and oxygen; exposingthe high-k gate dielectric layer to a solution that comprisestetramethyl ammonium hydroxide at a sufficient temperature for asufficient time to remove impurities from the high-k gate dielectriclayer and to increase the oxygen content of the high-k gate dielectriclayer; and then forming a gate electrode on the high-k gate dielectriclayer.
 2. The method of claim 1 wherein the gate electrode comprisespolysilicon.
 3. A method for making a semiconductor device comprising:forming a high-k gate dielectric layer on a substrate by atomic layerchemical vapor deposition, the high-k gate dielectric layer beingbetween about 5 angstroms and about 40 angstroms thick, and comprisingchlorine, oxygen, and a material selected from the group consisting ofhafnium oxide, zirconium oxide, titanium oxide, and aluminum oxide;exposing the high-k gate dielectric layer to a solution that comprisestetramethyl ammonium hydroxide at a temperature that is between about15° C. and about 90° C. for at least about one minute to remove chlorinefrom the high-k gate dielectric layer and to increase the oxygen contentof the high-k gate dielectric layer; forming a layer that comprisespolysilicon on the high-k gate dielectric layer; and etching thepolysilicon containing layer and the high-k gate dielectric layer.
 4. Amethod for making a semiconductor device comprising: forming a firsthigh-k gate dielectric layer on a substrate by atomic layer chemicalvapor deposition, the first high-k dielectric layer being between about5 angstroms and about 40 angstroms thick, and composing chlorine,oxygen, and a material selected from the group consisting of hafniumoxide, zirconium oxide, titanium oxide, and aluminum oxide; exposing thefirst high-k gate dielectric layer to a solution that comprises a sourceof hydroxide that is selected from the group consisting of deionizedwater, hydrogen peroxide, ammonium hydroxide, and a tetraalkyl ammoniumhydroxide, at a sufficient temperature for a sufficient time to removechlorine from the first high-k gate dielectric layer and to increase theoxygen content of the high-k gate dielectric layer; forming a secondhigh-k gate dielectric layer on the first high-k gate dielectric layer,and exposing the second high-k gate dielectric layer to a secondsolution that includes a source of hydroxide; forming a layer thatcomprises polysillicon on the second high-k dielectric layer; andetching the polysilicon containing layer, the first high-k gatedielectric layer, and the second high-k gate dielectric layer.